`<xstd/ints/bit/bit_repeat.hpp>`¶

`xstd::bit_repeat`¶

#ifdef XSTD_HAS_STD_BIT_PERMUTATIONS
// (1)
template<class T>
  requires ints::detail::std_permutes_bits<T>
  constexpr auto bit_repeat(T x, int l) -> T;
#endif

// (2)
template<unsigned_integer T>
  requires (not ints::detail::std_permutes_bits<T>)
  constexpr auto bit_repeat(T x, int l) -> T;

Repeats the low bits of an unsigned integer across its width.

The forwarding overload, which calls std::bit_repeat.
The template, for every other unsigned_integer.

Parameters¶

Parameter	Meaning
`x`	the value whose low bits are repeated
`l`	the period, the number of low bits repeated

Constraints: As for bit_reverse.

Preconditions: l is greater than zero.

Return value¶

Returns: repeat(x, l), P3104R5's sum of x[n mod l]·2^n over n in [0, N): the low l bits of x repeated across the width.

Exceptions¶

Throws: Nothing, for a T whose operators throw nothing.
It is not noexcept, for any type, the forwarding overload included.

Remarks¶

A call that violates the precondition is not a core constant expression, whether or not NDEBUG is defined, as P3104R6 [bit.permute]/8 adds.
Where __cpp_lib_bitops is at least 202607L and std::bit_compress, std::bit_expand, std::bit_repeat and std::bit_reverse all take T, the call is std::bit_repeat(x, l), and the template does not take T.
The template has no block kernel: it masks the low l bits and doubles the pattern until it fills the width, so it runs ceil(log2(N / l)) shift-or steps.

Notes¶

A period of one copies the lowest bit everywhere, and a period of N or more, INT_MAX included, returns x.
The paper's examples hold: bit_repeat(uint32_t{0xc}, 4) is 0xcccc'cccc, the alternating masks of its countr_zero are bit_repeat((1u << i) - 1, 2 * i), and that countr_zero agrees with xstd's.
A call is a constant expression for every type it takes.
The result has type T, not the int that integral promotion makes of a narrow T.

Entity	Relation
`bit_expand`	interleaves through masks it builds
`bit_reverse`, `bit_compress`	the other permutations
Notation	what `N` and `x[n]` mean here
P3104R6	the paper that proposes `std::bit_repeat`

<xstd/ints/bit/bit_repeat.hpp>¶

xstd::bit_repeat¶